U.S. Pat. No. 6,043,785 discloses an antenna according to the “Vivaldi” concept, comprising a dielectric strip with a metallization that forms an array of antenna elements. Each antenna element is formed by adjacent parts of the metallization with a tapered slot between these parts. The antenna element is fed with a field across the slot. An array of such antenna elements is formed by means of a series of such slots along the length of the dielectric strip.
It is known to form more complex arrays by placing a plurality of such strips in parallel to each other. An even more complex antenna may be formed with a grid of such strips, wherein a first plurality of mutually parallel strips is combined with a second plurality of mutually parallel strips, interlocking with the strips of the first plurality at right angles to the strips of the first plurality. Such an antenna is described in an article titled “A low profile wide-band (5:1) Dual-Pol Array” by J. J. Lee, S. Livingston and R Koenig in IEEE Antennas and Wireless Propagation Letters, 2, 2003.
This type of antenna provides for transmission and reception of radiation in a steerable beam direction with selectable polarization direction, and can be operated over a wide frequency bandwidth. The wide bandwidth is due to tapering of the slots. The selectable beam direction is controlled in transmission by the phase relation between the fields in the slots. The beam may be directed in a direction perpendicular to the rows and columns of the grid for example, by using fields of equal phase. When these fields are applied only to the slots of the strips of one of the pluralities of mutually parallel strips, a polarization direction parallel to these strips is obtained. A perpendicular polarization perpendicular to this can be obtained by applying the fields only to the slots of the other plurality of strips.
A disadvantage of this type of antenna is that it has been found to have relatively high cross-polarization, dependent on the direction in which the beam is steered. An array of such parallel strips causes an amount of polarization in a direction that does not correspond to the length of the strips. The polarizations generated by the rows and columns of the grid are not entirely orthogonal.